201 was one of the original area codes assigned in 1947, and was also the first area code to be assigned in the United States, as part of the North American Numbering Plan. It originally covered the entire state of New Jersey.[1][2] The decision to give only one area code to New Jersey was somewhat of a surprise at first, since it is the most densely populated state in the nation. It is split almost down the middle between two very large metropolitan areas--the New York metropolitan area in the north and the Delaware Valley metro area in the south. The North American Numbering Plan Administrator took this into account when it assigned 201 to New Jersey. Given the rotary dialing technology of the time, NANPA wanted to keep the number of "clicks" to a minimum for densely populated areas. Although codes with as few as five clicks were possible for an area code covering a portion of a state under NANPA's original guidelines, (0 and 1 were not allowed as the first digit, the second digit was either 0 or 1, and the third digit could not be the same as the second digit), area codes covering an entire state always had 0 as the second digit. New Jersey thus received an area code with 13 clicks, the fastest single-state area code that could be dialed under the original NANPA guidelines.

As the central region of New Jersey grew during the 1980s, the northeastern section of the state lost sizable chunks of its population due to the decline of its major cities, including Newark, Paterson, Clifton, and Elizabeth. On June 8, 1991, area code 908 split off from 201; it primarily serves the north-central regions of the state.[4]

Within four years, 201 was close to exhaustion once again due to the proliferation of cell phones and Internet dial-up connections. The supply of numbers was further limited because the entire northern half of the state is a single LATA, meaning numbers in 908 weren't available for use. On June 1, 1997, Essex and Passaic counties, home to Newark and Paterson respectively, the state's largest and third-largest cities, as well as Morris and Sussex counties, were broken away from area code 201 as area code 973.[4][5] This left Hudson and Bergen counties, the two most densely populated counties in the state and the closest to New York City, as the only counties in the 201 territory.

The 1997 split was intended as a long-term solution. However, demand for new numbers continued in Hudson and Bergen counties, particularly due to the proliferation of cell phones. It was debated whether 201 should be split again, or whether an overlay area code should be added. Overlays were a new concept at the time, and were somewhat controversial due to the need for ten-digit dialing. However, Verizon, the dominant telephone company in New Jersey, opted for an overlay to spare its customers the burden of having to change their numbers for the second time in a decade.

Area code 551 was created in 2001 to overlay 201, along with area codes 862 and 848, which overlay area codes 973 and 732, respectively.[6] With the implementation of the overlay area code on December 1, 2001, 10-digit dialing became mandatory.